Groups are nearly everywhere we look, but our ability to perceive them is anything but commonplace. Attention and memory have limited capacity, only allowing us to perceive and remember precise information about a few things at a time. To overcome these bottlenecks, the visual system engages a mechanism known as ensemble coding, in which noisy information about many items is condensed into a single representation that summarizes the group. This direct representation allows adults to appreciate the gist at little computational cost. Without it, the scope of our visual experience would be severely impoverished. Given the importance of ensemble coding, it is surprising how little is known about its development. While children can perceive basic properties of homogeneous groups, like numerosity, it is unclear if they possess the sophisticated ensemble algorithms necessary to summarize properties of heterogeneous groups, like average size. Here, we show that 4-5 year-old children do engage such summary mechanisms. Children viewed two groups of oranges or just a pair of oranges, all with a variety of sizes. When evaluating which group had the larger oranges overall, children integrated and averaged the sizes of multiple oranges, producing percepts of groups nearly as precise as those of single oranges. This pooling was independent of numerosity, continuous extent, density, and contrast. While sensitive, an ideal observer analysis showed that children integrated fewer oranges into their ensemble representations than adults. And with brief presentations, children engaged group perception with limited flexibility—those who excelled at ensemble coding were the worst at discriminating single oranges, and vice versa. Our findings provide a novel view of ensemble coding as it develops. More generally, they reveal new insights into the way children see and understand their environment, and they illustrate the fundamental nature of ensemble coding in visual perception.